1. Field of the Invention
The present invention relates to a photothermographic material.
2. Description of the Related Art
In recent years, decreasing the amount of processing liquid waste in the field of films for medical imaging has been desired from the viewpoints of protecting the environment and economy of space. Technology is therefore required for photosensitive thermal developing image recording materials which can be imagewise exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications. An image forming system using photosensitive thermal developing image recording materials does not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Photothermographic materials utilizing organic silver salts are described in many documents. Photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. The Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
A photothermographic material containing a photosensitive silver halide and a non-photosensitive organic silver salt is a material having high sensitivity, and is extremely favorable as an image recording material for laser output as described above, and it is expected that application thereof to this field will increase more and more in the future. In view of the expanding use in such fields of application and higher processing volumes, further increases in image recording speeds and developing speeds are desired. Improvement in performance of thermal developing processing, by shortening the time for processing, is a topic routinely demanded but it is particularly required in the medical field, in order to rapidly obtain photographed images and provide them to diagnosticians for rapid diagnosis.
As means for increasing the image forming speed, a method of increasing the sensitivity of a photosensitive material, to shorten the time for imagewise exposure, and a method of increasing the developing activity, to promote the thermal developing speed (increase of apparent sensitivity), can be mentioned. For improving the sensitivity of the photosensitive material, improvement of the photosensitive site of the silver halide is a direct method, and a sensitizing method is described in Japanese Patent Application Laid-Open (JP-A) No. 9-43765, the shape of silver halide grains is described in JP-A No. 2001-272743, and improvement for the silver halide composition is described in JP-A No. 9-146216. On the other hand, as a method of increasing the thermal developing speed, reducing agents are disclosed in JP-A No. 2001-188314, organic silver salts reduced by reducing agents are disclosed in JP-A No. 2000-72711, and use of development accelerators is described in JP-A Nos. 2002-156727 and 2001-264929. All patents, published patent applications, foreign applications, and non-patent literature listed in this specification are hereby incorporated by reference in their entirety.
An image forming layer is a direct element for forming images, and it is extremely important to consider compositions for use in the image forming layer as a method of improving the image forming speed. However, since such compositions are present in admixtures in the image forming layer, a conflicting phenomenon tends to occur whereby the storage stability deteriorates when the sensitivity or development activity is improved, whereas the sensitivity and the development activity are lowered when the storage stability is improved. It is extremely difficult to attain the performances described above simultaneously.
As described above, photothermographic materials are prepared in a well balanced manner so as to leverage the advantages of the respective compositions as much as possible and it is difficult to improve the image forming speed by merely changing or adding a single composition. Further, when a composition is changed or added, other compositions contained in the photothermographic material have also to be re-considered. A method of processing the photothermographic material rapidly without offsetting the features of respective compositions has been strongly demanded daily.